1. Field
The present invention relates generally to data communication, and more specifically to techniques for canceling interference due to pilots in a wireless (e.g., CDMA) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of systems, including increased system capacity. A CDMA system is typically designed to implement one or more standards, such as IS-95, cdma2000, IS-856, W-CDMA, and TS-CDMA standards, all of which are known in the art.
In some wireless (e.g., CDMA) communication systems, a pilot may be transmitted from a transmitter unit (e.g., a terminal) to a receiver unit (e.g., a base station) to assist the receiver unit to perform a number of functions. For example, the pilot may be used at the receiver unit for synchronization with the timing and frequency of the transmitter unit, estimation of the channel response and the quality of the communication channel, coherent demodulation of data transmission, and so on. The pilot is typically generated based on a known data pattern (e.g., a sequence of all zeros) and using a known signal processing scheme (e.g., channelized with a particular channelization code and spread with a known spreading sequence).
On the reverse link in a cdma2000 system, the spreading sequence for each terminal is generated based on (1) a complex pseudo-random noise (PN) sequence common to all terminals and (2) a scrambling sequence specific to the terminal. In this way, the pilots from different terminals may be identified by their different spreading sequences. On the forward link in cdma2000 and IS-95 systems, each base station is assigned a specific offset of the PN sequence. In this way, the pilots from different base stations may be identified by their different assigned PN offsets.
At the receiver unit, a rake receiver is often used to recover the transmitted pilot, signaling, and traffic data from all transmitter units that have established communication with the receiver unit. A signal transmitted from a particular transmitter unit may be received at the receiver unit via multiple signal paths, and each received signal instance (or multipath) of sufficient strength may be individually demodulated by the rake receiver. Each such multipath is processed in a manner complementary to that performed at the transmitter unit to recover the data and pilot received via this multipath. The recovered pilot has an amplitude and phase determined by, and indicative of, the channel response for the multipath. The pilot is typically used for coherent demodulation of various types of data transmitted along with the pilot, which are similarly distorted by the channel response. For each transmitter unit, the pilots for a number of multipaths of the transmitter unit are also used to combine demodulated symbols derived from these multipaths to obtain combined symbols having improved quality.
On the reverse link, the pilot from each transmitting terminal acts as interference to the signals from all other terminals. For each terminal, the aggregate interference due to the pilots transmitted by all other terminals may be a large percentage of the total interference experienced by this terminal. This pilot interference can degrade performance (e.g., a higher packet error rate) and further reduce the reverse link capacity.
Therefore, there is a need for techniques to cancel interference due to pilots in a wireless (e.g., CDMA) communication system.